1. Field of the Invention
The invention relates in general to a method for monitoring semiconductor integrated circuits (ICs), and more particularly to a method for monitoring dosage/focus/leveling at steppers while performing a photolithography process.
2. Description of the Related Art
Photolithography process plays an important role in semiconductor fabrications. For example, a metal oxide semiconductor (MOS) fabrication needs several photolithography processes to pattern several desired patterns and dope active regions. So, the number of photolithography processes taken usually represents the complexity of a fabrication process. The number of masks used also tells the complexity of a fabrication, because each photolithography process needs one mask.
According to reduction of the size of integrated circuits, process windows of a photolithography process, such as exposure energy (EE) and depth of focus (DOF), diminish. The baseline, such as exposure dosage, accuracy of focus or leveling of a chip, of a stepper must be controlled carefully during commercial production, so that an efficient monitoring system is required to ensure processes that are performed correctly.
A conventional method for measuring an exposure dosage is using a photo-speed monitor stepper to control the exposure dosage. When a photoresist layer on a wafer is exposed completely, that means the exposure dosage applied on the wafer is enough. The wafer is divided into several regions. Exposure dosage of a first region is lower than of a second region. Exposure dosage of the second region is lower than of a third region. The regions are checked one by one to determine whether the regions are developed. If some regions are not developed, the exposure dosage is insufficient. On the other hand, of regions are developed completely, the exposure dosage is sufficient. An exposure dosage of a stepper can be adjusted according to the results described above. However, to observe exposure is not easy due to nonuniformity of the photoresist layer and nonuniformity of developing.
Conventionally, a stepper laser beam is used to monitor the depth of focus of the stepper. Pattern length is measured under different defocus conditions according optical diffraction theory. The pattern length is the longest when the focus is correct. However, this process occupies the stepper for about 10 minutes.
A wafer placed on the stepper may be sloped so that the best focus is different at different position of the wafer. A conventional method uses an auto focus beam detect stepper to level the stepper stage. This process occupies the stepper for about 40 minutes.
These methods for monitoring dosage/focus/leveling are complicated and are performed sequentially. The methods require three wafers and parameters. Furthermore, the methods have to use a stepper so that they are unsuited for daily commercial production monitoring.